Integrated circuits and systems have continued to advance and become more complex at a rapid rate. As a result, effective and efficient power and thermal management of the integrated circuits and systems have become more and more critical in circuit design and implementation. In order to reduce the power consumption in integrated circuits and systems, these circuits and systems have been designed to operate at lower voltage levels. For example, integrated circuits and systems have been designed to operate at voltage levels such as 5 volts, 3.3 volts, or less. However, some components or circuitry in these integrated circuits or systems require higher voltages to operate or function. For instance, flash electrically erasable programmable read only (flash EEPROM) memory devices that are used in computers or systems typically require voltage levels that are higher than that provided by the power supply to perform various operations such as read, erase, or programming operations. In order to generate the voltage levels required by the flash memory that is higher than that provided by the power supply, charge pump circuits are typically used to generate a higher voltage level from a lower voltage level source. Charge pump circuits typically contain multiple pump stages that are used to increase a lower voltage input to a higher voltage output through incremental voltage increase at each stage. Each of the multiple pump stages in the charge pump circuits typically uses one or more capacitors for storing and transferring charge to the next pump stage in order to increase the voltage level from one stage to the next stage. In a typical charge pump circuit that includes multiple pump stages, each of the pump stages is to reach a sufficient voltage level (i.e., its own equilibrium voltage level) before the charge pump circuit can generate a steady current at the required output voltage. For example, assuming that there are three pump stages in a charge pump circuit that is designed to produce a required output voltage of Vout, each of the pump stages has to be pre-charged or warmed up to its equilibrium voltage level (V1 for the first stage, V2 for the circuit stage, and V3 for the third stage) before the charge pump circuit can generate a steady current at the required Vout voltage. Conventionally, the voltage levels of the various pump stages in a typical charge pump circuit are reduced to ground when the charge pump circuit is placed in a low power state (e.g., shut down, powered off, standby, etc.). More specifically, the various capacitors that are used for storing charge arc discharged as the charge pump circuit is put in the low power state (e.g., shut down, etc.). When the charge pump circuit transitions to a higher power state (e.g., active), all capacitors need to be pre-charged up to their equilibrium levels before the charge pump circuit can produce a steady current at the required output voltage level. It takes time and energy for the charge pump circuit to transition from a low power state (e.g., shut down) to a higher power state (e.g., running) as the capacitors cannot be pre-charged instantaneously from the ground level up to their equilibrium levels. The repeat of transitioning the charge pump circuit from one power state (e.g., shut down) to another power state (e.g., running) causes the charge pump circuit to waste an unacceptable amount of time and energy (i.e., charge) as the charge pump circuit needs to be warmed up from a low Vcc level.
Accordingly, there exists a need to reduce the amount of time and energy for charge pump circuits to reach their equilibrium levels when they are transitioned from a low power state to a higher power state.